Emulsion polymers free from emulsifiers and protective colloids, a process for their preparation and their use

ABSTRACT

The preparation of polymers free from emulsifiers and protective colloids by the free-radical initiated emulsion polymerization of ethylenically unsaturated copolymerizable monomers, wherein the emulsion polymerization is carried out in the absence of emulsifiers and protective colloids, using water-soluble, free-radical forming initiators and with the addition, at least at the start of the polymerization, of at least 0.01% by weight, based on the total amount of monomers, of poly-(ethylenically unsaturated) monomers, and the polymers can be obtained in the form of aqueous or non-aqueous dispersions or in the form of powder. 
     The use of the polymers in the form of dispersions or powders for modifying polymer materials and polymer raw materials and for the production of shaped articles and films.

This application is a continuation of Ser. No. 931,436, filed Nov. 12,1986 (now abandoned) which is a continuation of application Ser. No.767,244 (now U.S. Pat. No. 4,665,142), filed Aug. 20, 1985, which is acontinuation of application Ser. No. 600,325 (now abandoned), filed Apr.13, 1984.

The invention relates to emulsion polymers free from emulsifiers andprotective colloids, a process for their preparation and their use inthe form of dispersions or powders for modifying polymer materials orfor the production of shaped articles, especially film.

Aqueous polymer dispersions free from emulsifiers are already known (cf.European Patent A 0,054,685 and German Offenlegungsschrift 3,106,502). Afactor common to all of them is that, in order to achieve adequatestability at a high solids content, they require either the use ofprotective colloids or at least the use of at least one hydrophilicmonomer. In accordance with European Patent A 0,054,685 it is thereforepreferable to carry out polymerization with monomer fractions composedof glycidyl methacrylate, while in accordance with GermanOffenlegungsschrift 3,106,502 polymerization is carried out in thepresence of emulsifier acids which are soluble in organic solvents.

In accordance with European Patent A 0,054,685 polymerization is carriedout in a batch process, the entire quantity of monomer being chargedinitially. In this case, however, even if polyunsaturated monomers areconcomitantly used, only dispersions having a very low solids content(approx. 10% by weight) are obtained, and the monomer conversion isincomplete. In addition, carefully purification of the monomericstarting materials is necessary. The polymerization itself must becarried out under a protective gas. As a rule, however, it isadvantageous in the preparation of polymer dispersions to employformulations having fairly high solids contents in order, on the onehand, to achieve fairly high space-time yields and, on the other hand,to prevent, for example, excessively rapid deposition of the polymerparticles in the dispersion. In various applications of dispersions ofthis type it is necessary to remove the water they contain, and it is,of course, advantageous if the dispersion already contains as littlewater as possible et the outset. Furthermore, it i$ a disproportionateadditional expense for syntheses on a large industrial scale whenstarting materials have to be highly purified, as is required inaccordance with European Patent A 0,054,685.

When the dispersions are employed as systems or as mixing components formodifying polymer materials or other materials, such as, polymer rawmaterials for thermoplastic processing, the water-soluble and/orhydrophilic additives mentioned in German Offenlegungsschrift 3,106,502for stabilizing the dispersion can have a disadvantageous effect for awide variety of end uses, in that floating of the stabilizationadditives which are not fixed by a covalent bond can occur in thefinished product, as a result of which the properties of the product canbe severely impaired. For example, the heat stability or the thermalstability under load of polymer materials modified in this manner can beadversely affected by such extraneous substances which are not fixed bya covalent bond.

If dispersion polymers are used on their own it is also possible forsuch stabilizing ingredients, not fixed by a covalent bond, to affectthe polymer properties adversely and to exclude various applications.

The invention was therefore based on the object of developing emulsionpolymers which are free from emulsifiers and protective colloids, whichdo not have the disadvantages described above and which preferably havea narrow distribution of particle size, D_(w) /D_(n) ≦1.2, particularlywithin the particle diameter range between 0.02 μm and 5 μm. Thedispersions of such organic polymers should also, at a relatively highsolids content, be substantially free from any stabilization additiveswhich are not fixed by a covalent bond, and should, nevertheless, alsohave an adequate stability on storage and stability to heat.

Dispersions of organic polymer particles having average particlediameters between 0.02 and 5 μm can, for example, be employed veryadvantageously for modifying various materials, in particular polymermaterials. For example, it is essential in numerous applications oforganic polymer dispersions or polymers that the dispersions employed orthe polymers should be free from emulsifiers and protective colloids orshould in certain cases only contain negligible amounts thereof.Nevertheless, the dispersion must, however, be stable on storage and$table to heat and also free from residual monomers and insensitive tocoagulation or agglomeration when stored or when used in a system withwhich it is to be mixed.

On the other hand, pulverulent emulsion polymers which are free fromemulsifiers and protective colloids offer interesting fields of use, forexample in sectors hitherto reserved for suspension polymers,particularly sheets.

It is now possible, surprisingly, to achieve this object by means ofemulsion polymers or dispersions thereof which are obtained bypolymerizing ethylenically-unsaturated monomers in an aqueous system inthe absence of acyclic olefins with conjugated double bonds, e.g. di-and tri-(ethylenically unsaturated) compounds other than acyclicconjugated olefins, emulsifiers and protective colloids, usingfree-radical initiators which are partly or completely soluble in water,it being necessary, at least at the start of polymerization, for apoly-(ethylenically unsaturated) compound to be present in thepolymerization mixture in an amount of more than 0.01% by weight, basedon the total amount of monomer. In a preferred variant of the invention,it is possible to obtain thereby polymer dispersions which are free fromemulsifiers and protective colloids and have an average particlediameter between 0.02 and 5 μm at a narrow particle size distribution(D_(w) /D_(n) ≦1.2). Polymer dispersions which are particularlypreferred in this respect are those having particle diameters between0.05 and 2.0 μm and a particle size distribution D_(w) /D_(n) ≦1.05.

The particle size distribution D_(w) /D_(n) is a numerical measure forassessing the uniformity of particles in a dispersion. It is defined asthe ratio of the weight average of the particle diameter$ (D_(w)) andthe number average of the particle diameters (D_(n)). In the event ofcomplete uniformity it can assume the value 1. Non-uniformity ischaracterized by numerical values greater than 1 (cf. U.E. Woods et al.,Journal of Paint Technology, Volume 40, No. 527 (1968), page 545). Thedetermination of particle size can be effected by known methods, forexample by the electron microscope (cf. S.H. Maron et al., Journal ofAppl. Physics, Volume 23 (1952), page 900). It is thus possible, forcomparison purposes, to characterize a latex or a polymer dispersion inrespect of the uniformity of the particle size distribution by quoting asingle figure, namely the proportionality factor D_(w) /D_(n).

Corresponding non-aqueous polymer dispersions which are free fromemulsifiers and protective colloids can be obtained in accordance withthe invention either by carrying out the polymerization in a homogeneousmixture of water and a water-miscible non-aqueous phase as thedispersing medium, or by mixing the water-miscible, non-aqueous phasewith the aqueous dispersion when polymerization is complete, and ineither case subsequently removing the water more or less completely fromthe mixture. This can be effected, for example, by distillation. Vacuumdistillation is particularly preferred.

The emulsion polymers can be obtained from the aqueous polymerdispersions in e dry powder form free from emulsifiers and protectivecolloids by removal of water. The removal of water can be effected, forexample, by spray drying, freeze-drying or thin-film evaporation. Spraydrying is preferred.

The invention relates, therefore, to a process for the preparation ofpolymers free from emulsifiers and protective colloids by thefree-radical initiated emulsion polymerization of ethylenicallyunsaturated, copolymerizable monomers, which comprises first subjectingto preliminary polymerization, in the absence of emulsifiers andprotective colloids and using one or more water-soluble, free-radicalforming initiators in an aqueous emulsion, an amount or partial amountof the poly-(ethylenically unsaturated), copolymerizable monomer of atleast 0.01% by weight, preferably 0.01 to 20% by weight, particularlypreferentially 0.02 to 10% by weight and especially 0.1 to 5% by weight,based on the total amount of monomers, if appropriate on its own or,preferably, together with a partial amount of the mono-(ethylenicallyunsaturated) monomers of preferably 0.5 to 40% by weight, particularlypreferentially 1 to 30% by weight and especially 1.5 to 15% by weight,based on the total quantity of monomers, and then metering in theresidual amount of the mono(ethylenically unsaturated) monomer and, ifappropriate, the residual amount of the poly-(ethylenically unsaturated)monomer and, if appropriate, the residual amount of initiator,completing the polymerization of the mixture and, if appropriate,subsequently isolating the polymers from the resulting dispersion.

The content of disperse, polymeric solids in the resulting polymerdispersions is preferably 20 to 55% by weight, especially 25 to 45% byweight, based on the dispersion.

The process can be carried out discontinuously or continuously.

In accordance with the invention, the polymers are preferably in theform of a dispersion or a dry powder. They can, however, also constituteshaped articles, especially film.

Polymers according to the invention which are preferred are those inwhich the polymeric solid particles have a diameter within the rangefrom 0.02 to 5 μm, preferably 0.05 to 2.0 μm, and a narrow particle sizedistribution, so that the ratio of the weight average of the particlediameters (D_(w)) and the number average of the particle diameters(D_(n)) is preferably <1.2, particularly preferentially <1.1 andespecially <1.05.

The invention also relates to the polymers and polymer dispersionsprepared by the process according to the invention, it being possiblefor the dispersing medium in the dispersions to be aqueous and/ornonaqueous.

Suitable ethylenically unsaturated monomers are virtually all monomerswhich can be polymerized by free radicals, but the customary limitationsprescribed by the Q-diagram and e-diagram of Alfrey and Price and/or thecopolymerization parameters apply to copolymerization reactions (cf.,for example, Brandrup and Immergut, Polymer Handbook, 2nd edition(1975), John Wiley & Sons, New York) It should be pointed out explicitlythat, in general, the presence of hydrophilic monomers is not necessary,but is possible, for carrying out the polymerization reactions accordingto the invention.

The following are examples of types of ethylenically unsaturatedmonomers which can be employed. a) Monomers of the methacrylate type,preferably methyl methacrylate, n-butyl methacrylate, 2-ethylhexylmethacrylate, ethyl methacrylate, glycidyl methacrylate, hydroxyethylmethacrylate, hydroxypropyl methacrylate, glycidyl methacrylate,methacrylamide, N-methylolmethacrylamide, methacrylonitrile, tert.-butylmethacrylate and methacrylic acid, b) Monomers of the acrylate type,preferably ethyl acrylate, propyl acrylate, n-butyl acrylate,tert.-butyl acrylate, 2-ethylhexyl acrylate, acrylic acid,acrylonitrile, glycidyl acrylate and tert.-butyl acrylate, c) Monomersof the vinyl-aromatic type, preferably styrene, vinyltoluene andvinylpyridine, d) Monomers of the vinyl ester type, preferably vinylacetate, vinyl esters of branched (C₉ -C₁₂)-carboxylic acids, such asvinyl versatate, and vinyl linoleate, e) Monomers of the type of othermonocarboxylic and dicarboxylic acids, preferably crotonic acid, maleicacid, itaconic acid or esters or half-esters thereof, f) Monomers ofunsaturated halogen compounds, prefer vinyl chloride or vinylidenechloride, g) Monomers of the type of poly-(ethylenically unsaturated)compounds, e.g. die- and tri-(ethylenically unsaturated) compounds otherthan acyclic conjugated olefins, preferably divinylbenzene, ethanedioldiacrylate, ethanediol dimethacrylate, propanediol diacrylate,propanediol dimethacrylate, butanediol diacrylate, butanedioldimethacrylate, hexanediol diacrylate, hexanediol dimethacrylate,glycerol triacrylate, glycerol trimethacrylate, pentaerythritoltriacrylate, pentaerythritol trimethacrylate and diallyl phthalate.

The use of the poly-(ethylenically unsaturated) compounds mentionedunder (g) surprisingly makes it possible, inter alia, to achieve a highsolids content during the emulsion copolymerization, and, at the sametime, to achieve, in the finished dispersion, very good stabilityagainst coagulation of the batch. A content of at least 0.01% by weight,based on the total quantity of monomers, of one of the compoundsmentioned under (g) in the quantity of monomer initially charged isrequired for this purpose, at least during the starting phase of thepolymerization. The amount of poly-(ethylenically unsaturated) monomer,based on the total amount of monomers, should, in general, preferably bebetween 0.01 and 20% by weight, particularly preferentially 0.02 - 10%by weight and especially 0.02 - 5% by weight.

As a result of adding the poly-(ethylenically unsaturated) monomer whichacts as crosslinking agent, it is possible to prepare latices containingpartly or completely crosslinked particles which, at solids contents ofup to approx. 55% by weight, can have approximately monodisperseparticle size distributions and average particle diameters within therange from 0.02 to 5 μm. It is possible to prepare particles rangingfrom a $oft, elastic consistency to a hard, brittle, highly-crosslinkedstructure.

Examples of preferred monomer combinations are acrylate or methacrylatemonomers as mono-(olefinically unsaturated) compounds, anddivinylbenzene as a polyunsaturated compound. Examples of particularlypreferred combinations are those comprising methyl methacrylate, butylacrylate and divinylbenzene; or styrene, butyl acrylate anddivinylbenzene; or styrene, methyl methacrylate and divinylbenzene; ormethyl methacrylate, butyl acrylate, diVinylbenzene, methacrylic acidand/or acrylic acid; or styrene and divinylbenzene; or styrene,acrylonitrile and divinylbenzene; or vinyl acetate and butanedioldimethacrylate; or methyl methacrylate, butyl acrylate and butanedioldimethacrylate; or acrylonitrile and divinylbenzene; or vinyl chlorideand divinylbenzene.

The composition of the monomers should preferably be selected so thatthe content of hydrophilic monomers, such as, for example, acrylic acid,methacrylic acid, acrylonitrile, hydroxyethyl methacrylate, glycidylmethacrylate, maleic acid, maleic acid half-ester and the like is notmore than 25% by weight, preferably 15% by weight, based on the totalamount of monomers. The polymerization liquor is stirred at the stirringspeeds customary for emulsion polymerizations, for example at 10-200,preferably 3-100, r.p.m.

Dispersions, according to the invention, of organic polymer particlescan, for example, preferably be prepared in the following manner: 0.01to 10% by weight of a water-soluble, radical-forming initiator,preferably 0.02-5% by weight and particularly preferentially 0.05-3.0%by weight, based on the total amount of monomers in the batch, is addedto an aqueous system containing one or more types of ethylenicallyunsaturated monomers, of which 0.1-100% by weight, preferably 0.5-60% byweight and particularly preferentially 1-40% by weight, based on theamount of monomers initially, must be poly-(ethylenically unsaturated)monomers, and the polymerization is started, for example by thermolyticdecomposition of the free-radical initiator. Instead of initiallycharging the total amount of the initiator, it is also possible tocharge initially (in the aqueous liquor), only a fraction of theinitiator, preferably 1-90% by weight, particularly preferentially 5-70%by weight and especially 10-60% by weight, based on the total amount ofthe initiator, and to meter in the residual amount subsequently as anaqueous solution together with the still outstanding amount of monomers.

The proportion of monomers in the polymerization mixture initiallycharged, at the start of the polymerization reaction in mixture which isdesignated as preliminary polymerization, is preferably 0.01-30% byweight, particularly preferentially 0.1-25% by weight and especially1-20% by weight, based on the total amount, initially introduced, ofaqueous phase and non-aqueous monomer phase.

The duration of the preliminary polymerization is between 0.1 minute and3 hours, preferably 0.5 minute to 2 hours and particularlypreferentially 1 minute to 1 hour. The polymerization temperature isusually within a temperature range at which the initiator or theinitiator system has a half-life time of decomposition between 20minutes and 15 hours. After the completion of the preliminarypolymerization, the amounts of the monomer(s) and, when appropriate, ofthe residual initiator still outstanding are metered into thepolymerization batch. The rate of metering in the monomer(s) and, whenappropriate, the residual initiator should be adjusted to match thedecomposition rate of the initiator or the initiator system in such away that monomer does not float or settle out during the polymerization.

The emulsion polymerizations according to the invention are preferablycarried out within an acid pH range of <pH 7, preferably at pH 1 to 5.

The molecular weight of the polymers can be reduced in a known manner bythe use of molecular weight regulators. Mercaptans, halogen-containingcompounds and other radical-transferring substances are preferably usedfor this purpose. Butyl mercaptan, octyl mercaptan, dodecyl mercaptan,tetrakismercaptoacetylpentaerythritol, chloroform, carbon tetrachloride,trichloroethylene, trichlorobromomethane, bromoform and toluene areparticularly preferred. Water-soluble peroxides, azo compounds or redoxsystems are preferably employed as radical-forming initiators. Examplesof particularly preferred systems are Na, K or ammonium peroxydisulfateor Na, K or ammonium peroxydisulfate redox systems containing sulfides,sulfites or other reducing agents. Radical-forming initiators which arereadily and completely soluble in water are preferred. The amount of theradicall-forming initiator is preferably 0.01 to 10% by weight,particularly preferentially 0.02-5% by weight and especially 0.05-3.0%by weight, based on the total amount of monomers.

The particle size of the polymer particles can, inter alia, beinfluenced by the nature of the monomers used and their solubilitybehavior in water, and also by the nature and amount of thewater-soluble, radicalforming initiator used or by the mode in which itis metered in, and also by the nature and the amount used of thepoly-(ethylenically unsaturated) comonomers which result in polymericcrosslinking reactions, and also the nature and amount of themono-(ethylenically unsaturated) compounds which may be copolymerizedwith these comonomers in the preliminary polymerization and, in somecases, also in the main polymerization.

For some fields of use it can be advantageous to convert the aqueousdispersions obtained by the process according to the invention intonon-aqueous dispersions.

In order to obtain non-aqueous dispersions according to the inventionwhich are free from emulsifiers and protective colloids, either thepolymerization described aboVe i$ carried out in a mixture of water andan inert, non-aqueous organic compound, or, after polymerization hasbeen carried out in an aqueous phase, an adequate quantity of anon-aqueous organic compound or a mixture of such compounds is added andthe water is removed. These water-soluble organic compounds arepreferably organic compounds containing OH groups or mixtures of suchcompounds. Glycolic compounds or mixtures thereof with water-miscibleorganic compounds, or in some cases also with water-immiscible furtherorganic compounds, are particularly preferred. Thus dispersing mediacomposed of ethylene glycol or of mixtures containing ethylene glycolare particularly preferred for the use of the polymer dispersions forpigmenting, for example, polyester raw materials which can be used, forexample, for the production of sheets, fibers or filaments.

The water-miscible, non-aqueous dispersing medium or the water-miscible,non-aqueous, liquid phase preferably contains inert, water-solubleorganic compounds which contain hydroxyl groups and in which the OHgroups can be completely or partly substituted, preferably etherified oresterified. Compounds containing unsubstituted OH groups are preferred.Polyhydric alcohols, in particular dihydric alcohols, such as glycols orpolyglycols, and also polyglycol ethers in which the free hydroxylgroups can in some cases be substituted, are particularly preferred.

As inert, non-aqueous constituents, the dispersing medium can contain,for example, the following: monoalcohols, such as methanol, ethanol,isopropanol, butanol, amyl alcohols, iso-C₁₃ alcohol, lauryl alcohol,oleyl alcohol or butyldiglycol, monoesters or diesters of glycol orpolyglycols with lower carboxylic acids, ethylene glycol, propyleneglycol, glycerol, glycerol esters or partial esters, glycerol ethers,butylene glycol, phenol or alkylphenols. In removing the water of thedispersion by distillation, it is preferable to use non-aqueousconstituent$ having a boiling point higher than the boiling point ofwater. Ethylene glycol is particularly preferred.

Water is preferably expelled from mixtures containing ethylene glycol bydistillation under normal pressure or in vacuo. The use of entrainingagents or ternary mixtures can be particularly advantageous whenremoving water by distillation. The water content remaining in theresulting dispersion depends on the end use of the dispersion, forexample on the water-sensitivity of the system which is to be pigmentedwith the polymer particles. However, as the water content rises, boththe viscosity and the density of the resulting dispersion can fall,which in both cases promotes the tendency to settle out and thus canhave an adverse effect on the stability to storage.

The water content aimed at in the "non-aqueous" polymer dispersion is,therefore, preferably less than 20% by weight, particularlypreferentially less than 10% by weight and very particularlypreferentially less than 5% by weight, based on the dispersion.

Further fields of use for the polymers require the polymers to be usedin an isolated form, for example in the form of dry powders, which canbe obtained from the aqueous dispersion$, for example, by thin filmevaporation, freeze-drying or, especially, by spray drying. The polymerpowder which has been isolated in this manner can be incorporated intoother raw materials or products. It can, however, also be processed bycustomary methods, without further treatment, as a polymer raw materialsheets.

The invention is illustrated in greater detail by the examples below.Unless otherwise specified, the parts indicated in the examples areparts by weight. EXAMPLE 1

100 parts of a monomer mixture composed of 818 parts of methylmethacrylate, 441 parts of butyl acrylate and 25.2 parts ofdivinylbenzene (50% strength by weight) were added to 1,780 parts ofwater in a polymerization vessel equipped with a stirrer. The mixturewas heated to 80° C., and 51 parts of a solution of 2 parts of ammoniumpersulfate in 100 parts of water were added. After a preliminarypolymerization of 20 minutes, the remainder of the monomer was meteredin over the course of 7 hours. The remaining fractions of the initiatorsolution were added when the monomer had been completely metered in. Theduration of the subsequent reaction was 1 hour at 80° C.The dispersionprepared by this method had a solids content of 39.9% by weight, anaverage particle size (D_(w)) of 0.57 μm and a particle sizedistribution D_(w) /D_(n) of <1.1. The dispersion was stable andexhibited good stability to storage.

COMPARISON EXAMPLE 1

If the polymerization of Example 1 was carried out without the additionof divinylbenzene indicated in that example, the dispersion coagulatedeven before the monomer had been completely added.

EXAMPLE 2

50 parts of a monomer mixture composed of 400.5 parts of methylmethacrylate and 220.5 parts of butyl acrylate, and al$o 6.3 parts ofdivinylbenzene (50% strength by weight) were added to 886 parts of waterin a polymerization vessel equipped with a stirrer. The aqueous mixturewas heated to 80° C., and 25.5 parts of a solution of 1 part of ammoniumpersulfate in 50 parts of water were added. After 20 minutes, theremainder of the monomer was metered in over 7 hours. The remaining partof the initiator solution was added when the metering in was complete.The duration of the subsequent reaction was 1 hour at 80° C. Thedispersion prepared by this method had a solids content of 40% byweight, at a particle size distribution D_(w) /D_(n) of 1.05. Thedispersion wa$ $table and exhibited a good stability to storage. Theaverage particle size (D_(w)) was 0.33 μm.

COMPARISON EXAMPLE 2

If the polymerization of Example 2 was carried out without the additionof divinylbenzene indicated in that example, the dispersion coagulatedeven before the monomer had been completely added. EXAMPLE 3

724.6 parts of water, 6.3 parts of divinylbenzene (50% strength byweight) and 50 parts of a mixture of 315 parts of methyl methacrylate,315 parts of butyl acrylate, 12.6 parts of methacrylic acid and 6.3parts of acrylic acid were mixed together in a polymerization vesselequipped with a stirrer. The mixture was heated to 80° C., and asolution of 0.63 part of ammonium peroxydisulfate in 15 parts of waterwas added. After a preliminary polymerization of 20 minutes, theremainder of the monomer was metered in over 7 hours. Parallel withthis, 1.26 parts of ammonium peroxydisulfate, dissolved in 30 parts ofwater, were metered in. When the metered addition was complete, afurther 0.63 part of ammonium peroxydisulfate, dissolved in 15 parts ofwater, was added. The duration of the subsequent reaction was 1 hour at80° C.

The dispersion prepared by this method had a solids content of 45% byweight at a particle size distribution D_(w) /D_(n) of 1.05. Thedispersion was stable and exhibited a good stability to storage. Theaverage particle size (D_(w)) was 0.41 μm.

COMPARISON EXAMPLE 3

If the polymerization of Example 3 was carried out without the additionof divinylbenzene indicated in that example, the mixture coagulated evenbefore the monomer had been completely added.

EXAMPLE 4

765 parts of water were emulsified together with 6.3 parts ofdivinylbenzene (50% strength by weight), 21 parts of sodiumvinylsulfonate (30% strength by weight) and 50 parts of a monomermixture composed of 346.5 parts of styrene, 283.5 parts of butylacrylate, 12.6 parts of methacrylic acid and 6.3 parts of acrylic acidin a polymerization vessel equipped with a stirrer.

After the mixture had been warmed to 80° C., the polymerization wasinitiated by adding 5 parts of a solution of 25.2 parts of ammoniumperoxydisulfate in 60 parts of water and also 5 part$ of Na₂ S₂ O₅ in 60parts of water. Metered addition of the remainder of the monomer and theremainder of the redox initiator solution was started after 20 minutesand was carried out over 7 hours.

When the metered addition was complete, further solutions of 0.25 partof ammonium peroxydisulfate in 5 parts of water and 0.5 part of Na₂ S₂O₅ in 5 parts of water were added. The duration of the subsequentreaction was 1 hour at 80° C. The solids content of the dispersion was40.5% by weight at a particle size distribution D_(w) /D_(n) of <1.15.The dispersion was stable and exhibited good stability to storage. Theaverage particle size (D_(w)) was 0.47 μm.

COMPARISON EXAMPLE 4

If the polymerization of Example 4 was carried out without the additionof divinylbenzene indicated in that example, the dispersion coagulatedeven before the monomer had been completely added.

EXAMPLE 5

Reformulation using ethylene glycol as the dispersing medium.

360 parts of ethylene glycol were added to 600 parts of a dispersion,prepared in accordance with Example 1, in a vacuum distillationapparatus equipped with a rotary evaporator. 328 parts of water wereremoved by distillation from the mixture under a vacuum of 15-20 mm Hgand at an external temperature of 100-105° C. The resulting dispersionhad a solids content of 38% by weight.

We claim:
 1. A pulverulent organic polymer free from emulsifier and protective colloid and substantially free from any stabilization additive which is not fixed by a covalent bond to the polymer,the polymer being in dry-powder form or in finely-dispersed form in a liquid medium, the polymer comprising particles having a distribution of particle size D_(w) /D_(n) ≦1.2 within a particle diameter range between 0.02 μm and 5 μm, the polymer being a free-radically-initiated copolymerized emulsion copolymer, having units of monomer (a) and units of monomer (b), which is the same as that prepared by a polymerization comprising: a preliminary polymerization, in a polymerization medium, of poly-(ethylenically unsaturated) monomer (b) by itself or together with a partial amount of mono-(ethylenically unsaturated) monomer (a); the preliminary polymerization comprising that of at least 0.01 percent by weight, based on the total amount of monomers, of poly-(ethylenically unsaturated) monomer (b); metering into the polymerization medium remaining mono-(ethylenically unsaturated) monomer (a) or that together with any remaining amount of poly-(ethylenically unsaturated) monomer (b) and any remaining amount of initiator; completing polymerization of the resulting mixture to obtain a polymer dispersion; the mono-(ethylenically unsaturated) monomer (a) being a member selected from the group consisting of acrylic acid ester, methacrylic acid ester, a vinyl aromatic monomer, a vinyl ester, an ethylenically unsaturated monocarboxylic acid, an ethylenically unsaturated dicarboxylic acid, an ethylenically unsaturated halogen compound and sodium vinyl sulfonate; and the poly-(ethylenically unsaturated) monomer (b) being a member selected from the group consisting of divinylbenzene and butanediol dimethacrylate, the polymer having no more than 15% by weight of hydrophilic monomer units, based on the total amount of monomer units, and the amount of poly-(ethylenically unsaturated) monomer units being in the range of from 0.02 to %5by weight, based on the total amount of monomer units.
 2. A pulverulent polymer as claimed in claim 1 which is the same as that prepared by a polymerization comprising initially copolymerizing (a) with (b) in a polymerization reaction medium containing at least 0.01 percent by weight, calculated on the total monomer content, of (b).
 3. A polymer as claimed in claim 2 in finely dispersed form in a liquid water-miscible inert organic medium.
 4. A dispersion as claimed in claim 3 wherein the liquid medium is non-aqueous and comprises ethylene glycol.
 5. A polymer as claimed in claim 2 in dry-powder form.
 6. A pulverulent polymer as claimed in claim 1 which is the same as that prepared by a polymerization comprising initially polymerizing (b) by itself.
 7. A pulverulent polymer as claimed in claim 1, wherein the poly-(ethylenically unsaturated) monomer comprises divinylbenzene.
 8. A pulverulent polymer as claimed in claim 1, wherein the poly-(ethylenically unsaturated) monomer comprises butanediol dimethacrylate.
 9. A polymer as claimed inn claim 1 in finely dispersed form in an aqueous medium.
 10. A polymer as claimed in claim 1 which is partly or completely freed from the liquid dispersing medium.
 11. A polymer as claimed in claim 1, wherein the polymerization medium is free from emulsifier and from protective colloid, the polymerization has been initiated by at least one water-soluble, free-radical-forming initiator, andd has a dispersing medium which is water or an aqueous solution containing at least one water-miscible inert organic compound.
 12. A polymer as claimed in claim 11 wherein the dispersing medium is an aqueous solution containing an organic compound having hydroxy groups.
 13. A polymer as claimed in claim 11 wherein the dispersing medium is an aqueous solution containing ethylene glycol.
 14. A pulverulent organic polymer free from emulsifier and protective colloid and substantially free from any stabilization additive which is not fixed by a covalent bond to the polymer,the polymer being in dry-powder form or in finely-dispersed form in an aqueous or non-aqueous liquid medium, the polymer comprising particles having a distribution of particle size D_(w) /D_(n) ≦1.2 within a particle diameter range between 0.02 μm and 5 μm, the polymer being a free-radically-initiated copolymerized emulsion copolymer, having units of monomer (a) and units of monomer (b), which is the same as that prepared by a polymerization comprising b1) initially polymerizing crosslinkable poly-(ethylenically unsaturated) monomer (b), optionally with mono-(ethylenically unsaturated) monomer (a), in a polymerization reaction medium containing at least 0.01 percent by weight, calculated on total monomer content of said polymer, of (b) and 2) polymerizing the thus obtained product further with any residual amounts of (a) and (b) required to obtain the emulsion copolymer; the mono-(ethylenically unsaturated) monomer (a) consisting essentially of at least one member selected from the group consisting of acrylic acid ester, methacrylic acid ester, a vinyl aromatic monomer, a vinyl ester, an ethylenically unsaturated monocarboxylic acid, an ethylenically unsaturated dicarboxylic acid, an ethylenically unsaturated halogen compound and sodium vinyl sulfonate; the poly-(ethylenically unsaturated) monomer (b) being at least one member selected from the group consisting if divinylbenzene and butanediol dimethacrylate; (a) and (b) together having no more than 15% by weight of hydrophilic monomer units, based on the total amount of monomer units; and the amount of poly-(ethylenically unsaturated) monomer units being in the range of from 0.02 to 5% by weight, based on the total amount of monomer units. 